1. Field of the Invention
The present invention relates to a multi-user detection scheme in a direct sequence code-division multiple-access (DS-CDMA) system and, more particularly, to a method and apparatus for reducing the computations of a DS-CDMA receiver.
2. Description of Related Art
With the advent of cellular telephones and portable computers, wireless communication has become a preferred and increasingly prevalent method of communication. Multi-user detection used in wireless communication is directed toward the demodulation of digitally modulated signals in the presence of multi-access interference (MAI). Multi-user detection is most often applied in Code-Division Multiple-Access (CDMA) receiver designs. CDMA involves a method of multiplexing a group of users where each user is identified by a distinct code. Multiplexing allows any number of users the ability to transmit signals simultaneously, and allocates the entire available frequency spectrum to each user.
DS-CDMA is a popular technique of CDMA in which the transmitter multiplies each user's signal by a distinct code waveform. The receiver, due to an overlap of signals in time and frequency, detects a signal composed of the sum of all users' signals. The MAI component of the received signal is the result of random time offsets between the respective users' signals and makes the optimal design of orthogonal waveforms very difficult. In detecting a signal, a conventional DS-CDMA system uses an auto-correlation technique in which the entire received signal is correlated to a particular user's code waveform. This technique is limited because of a single-user detection strategy, which detects each user separately, not taking into account information within the signals of other users.
Multi-user detection strategies have been developed that use information of many users to better detect the coded signal of each individual user. These strategies are further described by Shimon Moshavi in “Multi-User Detection for DS-CDMA Communications”, IEEE Communications Magazine, October 1996, hereby incorporated by reference. Some prior art multi-user detection methods use techniques that model the received signal over the entire message. While this method captures a comprehensive interaction of users, the computations involve an ensemble of many bits, which introduces an abundance of excess processing. Other prior art methods involve partial correlation, which accounts for the partial overlap of bits within a channel. However, such methods require much iteration to compensate for signal loss due to interference.
In light of the limitations of the aforementioned prior art, there is a need for a DS-CDMA system that can effectively reduce the effects of signal interference when detecting the signal of a particular user within a multi-user system.